Earnings suppose that the rate of change f'(x) of the average annual earnings of new car salespersons is shown in the figure.
(a) If a, b, and c represent certain years, what is happening to f(x), the average annual earnings of the salespersons, at a, b, and c?
(b) Over what interval (involving a, b, or c) is there an increase in f(x), the average annual earnings of the salespersons?